I 52 naturally occurring inventory of uranium. Its subsequent transfer to man can be expected to be Tike that of uranium in nature. 2 Metabolic Pathways and Effects Uranium has a long history as a subject of toxicologic study. Hodge (1973), in reviewing the history of uranium poisoning fran 1824 to 1942, cited over 350 references. Later research was reviewed by Yuile (1973), who discussed experimentation in animals, and by Hursh and Spoor (1973), who summarized data on people. Most of the early sfudies, and the later ones as well, for that matter, emphasized the nephrotoxic effects of natural uranium. In general, hazards fran uranium were cons idered to relate more to its chemical toxicity than to its radioactivity. This is because uranfum-238, with a physical half life of 4.5.x 109- years, has a very low specific activity of about 0.33 microcuries per gram. However, there was concern that long-term exposures of sufficient duration, particularly enriched uranium, aight pose a radiation hazard, such as that which occurred in the luminous dial industry with the radium dial painters (Martland and Humphries, 1929; Martiand, 1931; Aub et al., 1952). Work tn the early 1940s (Tannenbaum et al., 1951) showed that tracer doses of uranium-233 as urany! nitrate injected subcutaneously into mice and dogs were primarily in the skeleton one and two months afterwards, respect ively. For mice, 67 percent of the total 233U in the body was in bone, while 14 Percent was in the kidney and |] percent in the liver. For dogs, 90 percent of the total burden was in bone, 3 percent in kidneys, and 3 percent in liver. That bone and kidneys were the principal sites of uranium accumulation was 81s0 found by Neuman et al. (1958).